1. Field of the Invention
The present invention relates to silicon crystallization, and more particularly, to a laser beam pattern mask and crystallization method using the same.
2. Discussion of the Related Art
As an information society develops, the demand for a visual display is rising in various ways. A variety of flat display devices have been designed to meet such a demand. Many efforts are now being made to further develop flat display devices, such as a liquid crystal display (LCD) device, plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Some of these flat display devices are already in use in various types of equipment.
When replacing CRT (cathode ray tube), LCD provides excellent features including excellent image quality, lightweight, compact size, and low power consumption. Thus increasing device portability and becoming the most popular device of the various flat display devices. Moreover, LCD has been also developed to be applicable to TV, a computer monitor, and the like as well as a portable notebook computer monitor.
A liquid crystal display device mainly consists of a liquid crystal display panel and a drive unit that applies a drive signal to the liquid crystal display panel. The liquid crystal display panel consists of first and second glass substrates assembled to each other to leave a predetermined distance from each other and a liquid crystal layer injected between the first and second glass substrates.
A plurality of gate lines are arranged in one direction on the first glass substrate (TFT array substrate) so as to have a predetermined distance between each other. A plurality of data lines are also arranged on the first glass substrate (TFT array substrate) having a predetermined interval between each other in a direction perpendicular to the gate lines. Further, a plurality of pixel electrodes are formed in pixel areas defined between the gate and data lines. Furthermore, a plurality of thin film transistors for transferring signals of the data lines to the corresponding pixel electrodes in response to signals of the gate lines are formed in the pixel regions, respectively. Such an arrangement is an active matrix LCD that provides excellent resolution and implementation of moving pictures by using pixel electrodes connected to thin film transistors.
A black matrix layer that only enables light to be transmitted to the pixel areas is formed on the second glass substrate (color filter substrate). In addition, an R/G/B color filter layer and a common electrode for implementing an image are formed on the second glass substrate (color filter substrate). The first and second glass substrates, between which is a predetermined space provided by spacers, are assembled to each other by a seal material having a liquid crystal injection inlet. Liquid crystals are then injected in the predetermined space via the liquid crystal injection inlet.
The operational principle of a general liquid crystal display device lies in making use of optical anisotropy and polarization characteristic of the liquid crystals. Liquid crystals have a thin and long shape that provides direction in the molecular arrangement. A direction of the molecular arrangement can be controlled by applying an electric field to the liquid crystals. A molecular arrangement direction of the liquid crystals can be adjusted such that the molecular arrangement of the liquid crystals is changed to refract light in the molecular arrangement direction of the liquid crystals by optical anisotropy to display image information.
In a liquid crystal display device including thin film transistors in which semiconductor layers are formed of polysilicon, the thin film transistors and a driver circuit can be provided on the same substrate so that the process of connecting the thin film transistors and the driver circuit is unnecessary, which simplifies the manufacturing process. Field effect mobility of polysilicon is 100˜200 times greater than that of amorphous silicon, and provides fast response speed and excellent stability in spite of high temperatures and light. Polysilicon can be formed by a high-temperature process or a low-temperature process. The high-temperature process requires a process temperature of about 1,000° C., which exceeds a transformation temperature of an insulating substrate. Hence, the high-temperature polysilicon process needs an expensive quartz substrate having high heat resistance. A polysilicon film formed by the high-temperature process shows low-grade crystalline properties, such as high surface roughness, fine grain size, and the like, thereby being inferior to polysilicon formed by the low-temperature process in terms of device operation characteristics. Hence, many efforts are being made to research and develop polysilicon formed by crystallization using amorphous silicon that can be deposited at low temperature.
The low-temperature process can be categorized into laser annealing, metal induced crystallization. Laser annealing is carried out in a manner of applying a pulse type laser beam to amorphous silicon on an insulating substrate. By using a pulse type laser beam, melting and solidification are repeated using 101˜102 nanosecond pulse such that damage to the lower insulating substrate is minimized. Much attention has been paid to laser annealing of amorphous silicon as a low-temperature crystallization process.
FIG. 1 is a graph of an amorphous silicon grain size according to laser energy density. Referring to FIG. 1, a crystallization area of amorphous silicon can be divided into first to third regions according to intensity of laser beam energy. The first region is a partial melting region, in which laser energy having intensity capable of partially melting a surface of an amorphous silicon layer is applied to the amorphous silicon layer. In the first region, a partial melting occurs just at the surface of the amorphous silicon layer after the laser beam irradiation. After solidification of the melted portions, small crystalline grains are formed on the surface of the amorphous silicon layer.
The second region is a near-complete melting region, in which laser energy is nearly able to melt the entire amorphous silicon layer is irradiated by increasing the laser energy intensity higher than that used in the first region. After melting, the grain grows larger than that of the first region by growing crystals from small nucleuses remaining as seeds. Yet, the second region fails to provide homogenous grains. Besides, the second region is considerably smaller than the first region.
The third region is a laser intensity sufficient to achieve complete melting, in which laser energy can entirely melt the amorphous silicon layer is irradiated by a laser energy intensity higher than that of the second region. Solidification proceeds after completion of melting the amorphous silicon layer, thereby enabling homogenous nucleation. After completion of irradiation, a crystalline silicon layer consisting of fine homogenous grains is formed.
To form large grains uniformly using the energy density of the second region, the number of irradiations of the laser beam and overlapping regions thereof are adjusted in the polysilicon forming process. Yet, a plurality of grain boundaries of polysilicon works as obstacles against a current flow, thereby inhibiting the production of a reliable thin film transistor device. A colliding current and degradation caused by collision between electrons within a plurality of the grains breaks down into an insulator, which results in product failure. To overcome such a problem, a single-crystalline amorphous silicon fabrication by SLS (sequential lateral solidification) has been proposed that is based on the fact that silicon grains grows on a boundary between liquid silicon and solid silicon in a direction vertical to the boundary (Robert S. Sposilli, M. A., Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc., Vol. 452, 956˜957, 1997).
In SLS, a silicon grain is laterally grown to a predetermined length by appropriately adjusting laser energy intensity, laser beam irradiated range, and laser beam translation distance such that amorphous silicon can be crystallized into single crystalline silicon longer than 1 μm (micrometer). Thus, a laser beam system used in the SLS focuses a laser beam on a narrow area, thereby unable to change all of the amorphous silicon on a substrate into single crystalline silicon at one time. Hence, to change a target area of the substrate, the substrate having an amorphous silicon layer deposited thereon is loaded onto a moving stage. After completely irradiating a predetermined area of the substrate, the substrate is moved to perform a next irradiation on a next area of the substrate. Thus, the irradiations are performed over all of the amorphous silicon on the substrate.
FIG. 2 is a schematic diagram of a general SLS irradiation system. Referring to FIG. 2, the SLS irradiation system consists of a laser beam generator (not shown in the drawing) generating a laser beam, an attenuator 1 decelerating the laser beam by adjusting energy intensity of the laser beam, a first mirror 2a changing the path of the laser beam, a telescope lens 3 spreading the laser beam, a second mirror 2b changing the path of the laser beam again, a homogenizer & condenser lens 4 uniformly condensing the laser beam, a third mirror 2C bending to change the path of the laser beam, a field lens 5 appropriately modifying a shape of the laser beam into an incident level of a laser beam pattern mask, a laser beam pattern mask 6 transmitting the laser beam into a predetermined pattern, and a projection lens 7 reducing the laser beam transmitted through the laser beam pattern mask 6 at a reduction ratio to irradiate the reduced laser beam on a substrate 10. The laser beam generator is an excimer laser typically using 308 nm XeCl or 248 nm KrF. The laser beam generator discharges a non-treated laser beam.
The discharged laser beam is passed through the attenuator 1, various mirrors 2a, 2b, and 2c to change its path and a laser beam pattern mask 6 so as to be irradiated on the substrate 10 with a predetermined laser beam pattern. The mirrors 2a, 2b, and 2c are provided to maximize the utilization of a space instead of elongating the space occupied by the SLS irradiation system in one direction. In some cases, the number of the mirrors can be adjusted to increase or decrease the space occupied by the SLS irradiation system.
A stage 8, on which the substrate 10 having an amorphous silicon layer deposited thereon is loaded, is placed in front of the laser beam pattern mask 6. As the substrate 10 is loaded on the stage 8 from outside, a fixing means for fixing the substrate 10 onto the stage 8 is provided to prevent the substrate 10 from moving on the movement of the stage 8. In order to crystallize the entire area of the substrate 10, the X-Y stage 8 is moved little by little to gradually extend a crystallized area.
The laser beam pattern mask 6 is divided into a transmitting part transmitting the laser beam and a cut-off part that cuts off the laser beam. A width of the transmitting part determines a lateral growth length of a grain formed by one exposure. The laser beam pattern mask 6 can have a pattern as explained in detail by referring to the attached drawings as follows. The present inventor has filed U.S. patent application Ser. No. 10/454,679 to a pattern for a lateral solidification and crystallization method of amorphous silicon, which is hereby incorporated by reference. Further, the present inventor is also an inventor in U.S. Pat. Nos. 6,755,909, 6,736,895,  6,489,188 and 6,326,286, which are also hereby incorporated by reference.
FIG. 3 is a layout of a laser beam pattern mask used for laser irradiation, and FIG. 4 is a diagram of crystallized regions formed by a first laser beam irradiation using the laser beam pattern mask in FIG. 3. Referring to FIG. 3, a laser beam pattern mask 3 used in laser irradiation consists of a plurality of transmitting parts ‘A’ having open patterns with a first interval ‘a’, respectively and a plurality of cutoff parts ‘B’ having closed patterns with a second interval ‘b’, respectively. The transmitting and cutoff parts ‘A’ and ‘B’ are alternately provided to the laser beam pattern mask 3.
A laser beam irradiation method using the laser beam pattern mask is explained as follows. First, a laser beam is irradiated onto a substrate having an amorphous silicon layer deposited thereon using the laser beam pattern mask 3. In doing so, the laser beam passes through a plurality of the transmitting parts ‘A’ provided on the laser beam pattern mask 3. A portion 22, which corresponds to the each of the transmitting parts ‘A’, is melted into a liquid phase. Thus, the intensity of the laser energy used is in the complete melting region, that is, enough energy to completely melt the irradiated portion 22 of the amorphous silicon layer. An irradiated region of the substrate, such as the area defined by width ‘L’×length ‘S’ corresponding to a region of the laser beam pattern mask where a plurality of the consecutive transmitting parts ‘A’ are formed, is called a unit region 20.
After completion of the laser beam irradiation, lateral growth of silicon grains 24a and 24b proceeds from interfaces 21a and 21b between an amorphous silicon region and the liquefied silicon region after complete melting. The lateral growth of the grains 24a and 24b occurs in a direction vertical to the interfaces 21a and 21b. If a width of the irradiated portion 22 corresponding to the transmitting part ‘A’ is smaller than double the growth length of the crystallized silicon grain 24a, the grains vertically grown inward from both of the interfaces 21a and 21b with the amorphous silicon region and the irradiated portion 22 collide with each other and stop growing.
To further grow the silicon grains, the stage having the substrate loaded thereon is moved so that another irradiation is performed on a neighboring region adjacent to the irradiated portion to form crystals connected to the former crystals formed by the first irradiation. Likewise, crystals are laterally formed from both sides of an irradiated portion are instantly melted by the second irradiation. Generally, a length of the crystalline growth connected to the neighbor irradiated portion by laser irradiation depends on widths of the transmitting and cutoff parts ‘A’ and ‘b’ of the laser beam pattern mask.
FIG. 5 is a layout of a crystallization sequence of a process according to a related art. Referring to FIG. 5, silicon crystallization proceeds on a substrate 10 according to a sequence shown in FIG. 5.
A first translation T1 is made in the following manner. An exposed portion of the substrate 10 is moved to a left side from a right side (in a (−)X-axis direction) to correspond to the transmitting part beam length ‘L’ of the laser beam pattern mask 6 in FIG. 3. An irradiation is then performed in a horizontal direction of the substrate 10.
Subsequently, a second translation T2 is made in the following manner. The exposed portion of the substrate 10 is moved to a lower side from an upper side (in a (−)Y-axis direction) to correspond to a ½ width [(a+b)/2] of the transmitting and cutoff parts ‘a’ and ‘b’.
A third translation T3 is then made in a following manner. The exposed portion of the substrate 10 is moved to the right side from the left side (in a (+)X-axis direction) to correspond to the transmitting part beam length ‘L’. And, crystallization is performed on each region, on which the irradiation after the first translation T1 fails to be performed, of the unit regions of the substrate 10 corresponding to the laser beam pattern mask 6. In doing so, the laser beam irradiated region of the third translation T3 is overlapped in part with that of the first translation b so that crystals can continue to grow from the former crystals.
Subsequently, a fourth translation T4 is made in a following manner. The exposed portion of the substrate 10 is moved to the lower side from the upper side (in a (−)Y-axis direction) to correspond to the vertical length ‘S’ of the laser beam pattern mask 6. And, the processes of the first to third translations T1 to T3 are repeated to perform crystallization on the entire surface of the substrate 10. In the above-explained crystallization, the stage 8 in (FIG. 2) having the substrate 10 loaded thereon is moved to perform the crystallization while the laser beam pattern mask 6 is fixed. In doing so, the corresponding translation of the stage 8 is made in a direction opposite to the crystallizing direction on the substrate 10.
FIG. 6 is a diagram of beam overlap occurring on a predetermined area after completion of crystallization across an entire area of a substrate according to the related art. Referring to FIG. 6, when crystallization makes progress by moving the substrate 10 according to the sequence in FIG. 5, unit regions C1, C2, . . . Cm, Cm+1, . . . of the substrate corresponding to one laser beam irradiation are moved to perform crystallization. Looking into a predetermined area of the substrate after completion of the crystallization, beam overlap areas 01 and 02, where the transmitting part ‘A’ corresponds at least twice between adjacent laser irradiation areas, take place. Namely, the beam overlap area 01 takes place when irradiation is performed after the X-axis directional translation of the substrate corresponding to the length ‘L’ of the laser beam pattern mask 6. And, the beam overlap area 02 takes place when irradiation is performed after the Y-axis directional translation of the substrate corresponding to the length ‘(a+b)/2’ of the laser beam pattern mask 6. There are doubly exposed portions 51 and 52 overlapped in one direction of X- or Y-axis. And, there is a quadruply exposed portion 53 overlapped in both directions of the X- and Y-axis.
The laser beam is irradiated at least twice on the beam overlap area 01 vertical to a crystalline orientation direction among the beam overlap areas 51, 52, and 53. In doing so, the formerly formed grains are melted in part to be crystallized again, whereby the final grains are formed tilted. If a device is placed on such a tilted portion, mobility is lowered so as to degrade the characteristics of a device later formed on the portion.
FIG. 7 is a SEM picture representing a degree of crystallization appearing on a normal or beam overlap area according to the related art. Referring to FIG. 7, compared to a crystallization degree of a normal area, a crystallization degree appearing in a beam overlap area of adjacent laser beam pattern areas on crystallization has an irregular characteristic. Specifically, grains in the beam overlap area are formed in a tilted direction, whereas grains of the normal area are formed in a vertical direction. When patterning is performed so that devices of one line are formed on a normal area in part and that the rest of the devices are formed on one of the beam overlap areas, devices characteristics for each of the devices will be inconsistent.
As mentioned in the foregoing description, the related art laser beam pattern mask and crystallization method using the same have the following problems or disadvantages. First of all, in irradiating the laser beam onto the substrate using the laser beam pattern mask, beam overlap areas occur between the adjacent laser beam pattern area. More specifically, edge portions of adjacent laser beam irradiation pattern areas overlap with each other and the shape of the laser beam irradiation pattern area is like a shape of the transmitting part provided in the laser beam pattern mask. Yet, center and edges of the transmitting part of the related art laser beam pattern mask are rectangular, whereby the beam overlap area on the substrate using the mask has a rectangular shape as well. The beam overlap area has crystallization characteristics that are irregular as compared to the non-overlapped area. Specifically, in fabricating a thin film transistor array substrate to manufacture a flat panel display such as a liquid crystal display device, device characteristics can be changed such that operational malfunctions occur in a thin film transistor (TFT) located on the beam overlap area.